1. Field of the Invention
The present invention relates to a magnetic disk apparatus having a disk rotating motor.
2. Description of the Related Art
Recently, the size of a magnetic disk apparatus has been reduced more and more, and 2.5-inch magnetic disk apparatuses have been widely used in place of 3.5-inch magnetic disk apparatuses. In addition, 1.8-inch magnetic disk apparatuses have newly been introduced into the market.
As the diameter of a disk used in the magnetic disk apparatus is decreased, the width, length and height of the magnetic disk apparatus must be reduced accordingly. When the size of the apparatus is reduced, it is necessary to reduce the size of each part of the apparatus, while maintaining the required characteristics of the mechanisms of the apparatus. Furthermore, since the small-sized magnetic disk apparatus is used, for example, in a lap-top personal computer under relatively harsh conditions, it is important that the apparatus have good vibration-resistant properties and shock-resistant properties at the time of operation and non-operation.
Under the circumstances, in a small-sized magnetic disk apparatus, in particular, the vibration (resonance) properties of the mechanism of the apparatus in a low frequency range (e.g. about 500 Hz) are important.
In a simple model, the resonance frequency F is expressed by ##EQU1##
In the small-sized magnetic disk apparatus, the weight of each part is reduced, but the rigidity of the apparatus is degraded owing to the reduction in thickness of the apparatus. Thus, the resonance frequency of the entire apparatus reduces.
Where the resonance frequency in the direction of a motor shaft is low and the vibration transmission gain is high, a resonance phenomenon occurs when the apparatus is actuated and vibration of the disk is increased. Since a magnetic head performs read/write operations, while flying over the disk about 0.1 .mu.m, the increase in vibration of the disk degrades the compliance characteristics of the magnetic head over the disk. In other words, the electromagnetic conversion performance is degraded.
When a resonance has occurred in a direction of the surface of the disk (a direction perpendicular to the motor shaft), the disk generally vibrates at a so-called non-synchronized frequency, which is near a resonance frequency which is different from an integer-number of times of the rotation frequency. Thus, the positioning precision of the magnetic head deteriorates and the apparatus performance lowers considerably.
In addition, in general, the operational vibration of the magnetic disk apparatus according to specifications is about 500 Hz at most. If the resonance frequency is about 500 Hz or less, the vibration characteristics against external disturbance during operation is degraded.
As regards a spindle motor, the material of a hub is aluminum, magnetic SUS (stainless steel), or free-cutting steel, whereas the material of a bracket is aluminum, e.g. die-cast aluminum. FIG. 5 shows a simplified model of axial rigidity where the hub rigidity is k1, bearing rigidity is k2 and bracket rigidity is k3, the motor rigidity K and resonance frequency F are expressed by EQU K=k1.k2.k3/(k2.k3+k1.k3+k1.k2) (2) EQU F=(1/2.pi.).(K/m).sup.1/2 ( 3)
Accordingly, in order to increase the resonance frequency, it is necessary to reduce the mass m or increase the hub rigidity k1, bearing rigidity k2 or bracket rigidity k3. In the case of the small-sized spindle motor, the bearing rigidity k2 is higher than the hub rigidity k1 or bracket rigidity k3. Thus, in order to solve a problem of vibration (rigidity), it is necessary to increase the hub rigidity k1 and/or bracket rigidity k3.
The bracket rigidity k3 can be increased by increasing the thickness of the bracket, but the limitation to the thickness dimension is strict in the small-sized magnetic disk apparatus. Thus, the thickness of the bracket cannot be greatly increased.